US4519089A - System for detecting neutral state of a transmission of an engine for vehicles - Google Patents

Abstract

A system for detecting a neutral state of a transmission for an engine mounted on a vehicle comprises an engine speed detecting device for producing an engine speed output signal in proportion to the engine speed; and a vehicle speed detecting device for producing a vehicle speed output signal in proportion to the vehicle speed. An operation circuit is provided for judging whether the ratio of the engine speed output to the vehicle speed output is in a transmission ratio range dependent on the transmission. The operation circuit is so arranged as to produce at least one neutral state output when the ratio of the engine speed output to the vehicle speed output is not in the transmission ratio range. The operation circuit comprises counters for counting the output pulses, circuits for producing signals for deciding the period of operation of the counters, and circuits including a gate circuit for producing the neutral state output in dependency on the result of the counting during the period of the counter operation.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a system for detecting the neutral state of a transmission for an engine for vehicles.

Heretofore, an engine speed control system has been provided for controlling the engine speed. The system is, for example, adapted to control idling speed to a desired idling speed by adjusting the amount of air or the amount of the air-fuel mixture to be induced in the engine in dependency on an error signal which is the difference between a desired reference idling speed and the detected idling speed. In such an automatic, engine speed control system, the detecting of the neutral state of a transmission is necessary.

A conventional neutral state detecting system comprises a clutch switch provided adjacent to a clutch pedal of the vehicle and a neutral switch provided in the transmission. The neutral state detecting system operates to judge the neutral state of the transmission from two signals obtained by the above described switches for producing a neutral signal.

However, since the neutral switch is provided in the transmission, the switch is complicated in construction and expensive.

The object of the present invention is to provide a neutral state detecting system which reliably detects the neutral state and which has a simplified construction.

According to the present invention, there is provided a system for detecting the neutral state of a transmission for an engine mounted on a vehicle comprising an engine speed detecting means for producing an engine speed output signal proportion to the engine speed; a vehicle speed detecting means for producing a vehicle speed output signal in proportion to the vehicle speed; an operation circuit for judging whether the ratio of the engine speed output to the vehicle speed output is in a transmission ratio range dependent on the transmission; the operation circuit being so arranged as to produce at least one neutral state output when the ratio is not in the transmission ratio range.

Other objects and features of the present invention will be fully described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a system of the present invention;

FIG. 2 is a perspective view showing a carburetor and attachments thereof;

FIG. 3 is a front view of an actuator for the carburetor;

FIG. 4 is a side view of the actuator;

FIG. 5 is a perspective view showing the internal construction of the actuator;

FIG. 6 is a sectional view showing a vehicle speed sensor;

FIG. 7 is an electric circuit of an engine speed control circuit employed in the system;

FIG. 8 is an electric circuit of a neutral state detecting circuit of the present invention;

FIG. 9 is a graph showing outputs of comparators in the control circuit of FIG. 7; and

FIG. 10 is a graph for explaining the principle of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, aninternal combustion engine 1 mounted on a vehicle (not shown itself) is provided with anintake manifold 2, acarburetor 3, anair cleaner 4, anignition coil 5, and spark plugs 6 connected to the ignition coil through a lead 7, respectively. Anactuator 8 for operating a throttle valve 18 (FIG. 2) in the carburetor is supported on the side wall of thecarburetor 3. Theactuator 8 includes an idling sensing switch which is hereinafter described. Idling signals produced by the idling sensing switch in theactuator 8 are sent to acontrol circuit 10 by alead 9. Pulses produced in synchronism with ignition pulses are also supplied to thecontrol circuit 10 through alead 11. The output of thecontrol circuit 10 is connected to theactuator 8 by alead 12.

Avehicle speed sensor 13 is connected to an axle on afront wheel 14 of the vehicle by aspeedometer cable 15. The output of thevehicle speed sensor 13 is applied to thecontrol circuit 10 by alead 16.

Referring to FIG. 2, athrottle shaft 17 of thethrottle valve 18 is rotatably supported in thecarburetor 3, one end of thethrottle shaft 17 being secured to the inner end of aspring 19 which exerts a spring force on thethrottle shaft 17 biasing thethrottle valve 18 to close. Awire connector 20 and alever 21 are secured to thethrottle shaft 17 at the other side of the throttle shaft. An end of anaccelerator wire 22 connected to an accelerator pedal (not shown) of the vehicle is fixed to thewire connector 20 and an end of arod 23 of theactuator 8 is located adjacent to an end portion of thelever 21. Referring to FIGS. 3 and 4, theactuator 8 is secured to asupport 24 secured to the wall of thecarburetor 3 and the end of therod 23 is adjacent to abent end portion 25 of thelever 21. The end of therod 23 and theend portion 25 cooperate to act as the idling sensing switch 106 (FIG. 7) as will be hereinafter described.

FIG. 5 shows the internal construction of theactuator 8. Therod 23 is rotatably supported by abearing 27. Therod 23 is formed with athread 26 which is engaged with a screw formed in abearing 28. Therod 23 is secured to agear 32. Although the rod is made of metal, thebearings 27, 28 and thegear 32 are made of plastic. Accordingly, the rod is insulated from the body of theactuator 8. Aterminal 30 is secured to the body of the actuator through aninsulating plate 29. Abrush 31 is secured to theterminal 30 and elastically engaged with the periphery of therod 23.

Thegear 32 engages with a pinion 33 securely mounted on a shaft 34, the end of which has agear 35 secured mounted thereon. Thegear 35 is engaged with a pinion 36 secured to a shaft 37 of amotor 38.

Referring to FIG. 6 showing thevehicle speed sensor 13, a plurality ofpermanent magnets 39 are circumferentially arranged on ashaft 42 connected to thespeedometer cable 15 and secured thereto. Themagnets 39 with their polarities as indicated are separated, for example, from each other bysynthetic resin 41 or other insulation secured on theshaft 42. Areed switch 40 is provided on asupport 43 and positioned in the vicinity of themagnets 39 with a slightly spaced therefrom forming a gap therebetween. Thus, thereed switch 40 is turned on and off during the rotation of thespeedometer cable 15.

Referring to FIG. 7, thecontrol circuit 10 generally comprises an engine speedsignal generating circuit 50, an engine speed increasingsignal generating circuit 51, and an engine speed decreasingsignal generating circuit 52,oscillators 53 and 54, anactuator control circuit 55, a vehicle speedsignal generating circuit 56, an actuatorsignal generating circuit 57, and anAND gate 59 as a control gate.

The engine speedsignal generating circuit 50 is applied with pulses from theignition coil 5, which causes the switching of a transistor switchingcircuit comprising resistors 60, 61, 62 and atransistor 63. The collector of the transistor is connected to a positive voltage source through aresistor 64 and to an integratingcircuit comprising resistors 65, 67 and acapacitor 66.

The output of the engine speedsignal generating circuit 50 is connected to a non-inverting input of thecomparator 70 of the engine speed increasingsignal generating circuit 51 and connected to an inverting input of acomparator 75 of the engine speed decreasingsignal generating circuit 52. The inverting input of thecomparator 70 is connected to a voltagedivider comprising resistors 68 and 69 and the output thereof is connected to anAND gate 71. The output of theAND gate 71 is connected to anAND gate 72 for producing an engine speed increasing signal. The non-inverting input of thecomparator 75 is connected to a voltagedivider comprising resistors 73 and 74, and the output thereof is connected to anAND gate 76. The output of theAND gate 76 is connected to anAND gate 77 for producing an engine speed decreasing signal.Oscillators 53 and 54 compriseinverters 78, 79, 83 and 84,resistors 80, 81, 85 and 86, andcapacitors 82 and 87, respectively. Pulses from theoscillator 53 are applied to theAND gate 71 and pulses from the oscillator 54 are applied to theAND gate 76. ANDgates 72 and 77 are applied with an output of theAND gate 59.

Theactuator control circuit 55 for engine speed control hasPNP transistors 88 and 89,NPN transistors 90 and 91 which are connected with each other in bridge form. Themotor 38 is connected between collectors of two pairs of the transistors. Theactuator control circuit 55 further comprises a pair oftransistors 98 and 99 andresistors 92 to 97 for applying voltages to each transistor. The base of the transistor 98 is applied with an output of theAND gate 72, and the base of thetransistor 99 is applied with an output of theAND gate 77.

The output of thevehicle speed sensor 13 comprising thereed switch 40 is connected to a positive voltage source via aresistor 110, and to an EXCLUSIVE-OR gate 111, and to aninverter 112 of the vehicle speedsignal generating circuit 56. The EXCLUSIVE-OR gate 111 and theinverter 112 form a one-shot multivibrator together with aresistor 113, aninverter 114 and a capacitor 115. The output of the EXCLUSIVE-OR gate 111 is connected to the inverting input of acomparator 103 via aresistor 101. Between the inverting input and the ground, acapacitor 102 and aresistor 160 are connected. The non-inverting input of thecomparator 103 is connected to a voltagedivider comprising resistors 104 and 105. The output of thecomparator 103 is connected to one of the inputs of the ANDgate 59. Between the non-inverting input of thecomparator 103 and the ground, ananalogue switch 116 and aresistor 117 are connected in series. The control gate of theanalogue switch 116 is connected to the output of a NORgate 118, which will be described hereinafter.

The actuatorsignal generating circuit 57 includes the actuator contact idlingsensing switch 106 which is formed by the end of therod 23 and theend portion 25 of thelever 21. An end of theswitch 106 is grounded and the other end is connected to the voltage supply through aresistor 108 and to a base of atransistor 109. The collector of thetransistor 109 is connected to the voltage supply through aresistor 107 and to the input of ANDgate 59.

Referring to FIG. 8 showing a neutral state detecting circuit, an upper circuit portion of the circuit is an operation circuit for the neutral state of zone A and a lower circuit portion is an operation circuit for the neutral state of zone B. The output of theignition coil 5 is connected to awaveform shaping circuit 161. Thewaveform shaping circuit 161 comprises atransistor 165 andresistors 162, 163, 164 and 166. The output of thewaveform shaping circuit 161 is connected to a clock input C of acounter 120. Outputs Q₁ and Q₂ and Q₃ of thecounter 120 are applied to an ANDgate 122 of a first neutral state judging circuit 121. The output of the ANDgate 122 is connected to an S input of a flip-flop 123, the Q output of which is in turn connected to an S input of a flip-flop 124. The Q output of the flip-flop 123 is connected to the R input of flip-flop 124 through an ANDgate 125.

Thereed switch 40 connected to the positive source via aresistor 132 is connected to a clock input C of acounter 126. Outputs Q₀, Q₁ of thecounter 126 are connected to an S input of a flip-flop 129 through an ANDgate 128 of a second neutral state judging circuit 127. The Q output of the flip-flop 129 is connected to an S input of a flip-flop 130 and the Q input of the flip-flop 129 is connected to the R input of the flip-flop 130 through an AND gate 131.

Thereed switch 40 is also connected to a one-shot multivibrator 133. The one-shot multivibrator 133 comprises an ANDgate 134,inverters 135, 137,resistor 136, and capacitor 138. The output of the ANDgate 134 is connected to the R input of thecounter 120 and to the R input of the flip-flop 123 through aninverter 139 and a capacitor 140. Further, the output of the ANDgate 134 is connected to the ANDgate 125 through acapacitor 141. Both of thecapacitors 140, 141 are grounded throughresistors 142, 143, respectively.

The output of thewaveform shaping circuit 161 is also connected to a clock input C of a J-K flip-flop 144. The Q output of the flip-flop 144 is connected to an ANDgate 146 and to aninverter 147 of a one-shot multivibrator 145. The one-shot multivibrator 145 comprises aresistor 148,inverter 149 and a capacitor 150 in addition to the ANDgate 146 and aninverter 147. The output of the one-shot multivibrator 145 is connected to the R input of thecounter 126 and to the R input of the flip-flop 129 through aninverter 151 and acapacitor 152. Further, the output of the one-shot multivibrator 145 is connected to the AND gate 131 via a capacitor 153.Capacitors 152 and 153 are grounded viaresistors 154 and 155, respectively.

In operation, referring to FIG. 7, pulses proportional to ignition pulses of the engine are applied to the engine speedsignal generating circuit 50. Rotation of thefront wheel 14 causes the shaft 42 (FIG. 6) of thevehicle speed sensor 13 to rotate and to generate output pulses in proportion to the vehicle speed on the end of thereed switch 40. As to the actuator contact idling sensing switch 106 (in circuit 57), if the accelerator pedal is depressed for acceleration of the engine, theend portion 25 of thelever 21 secured to thethrottle shaft 17 is disengaged from the end of therod 23, which means opening of the idlingsensing switch 106.

Pulses applied to the engine speedsignal generating circuit 50 turn on and off thetransistor 63. The voltage on the end of thecapacitor 66 varies in inverse proportion to the engine speed. The voltage at thecapacitor 66 is applied tocomparators 70 and 75. When the input voltage of thecomparator 70 is higher than the lower limit reference voltage at the inverting input, which means a low engine speed, a high level output signal is applied to the ANDgate 71. The ANDgate 71 produces pulses according to the input pulses from theoscillator 53, which are applied to the ANDgate 72. When the input voltage of thecomparator 75 decreases below the upper limit reference voltage at the non-inverting input, which means the engine speed becomes high, a high level output signal is applied to the ANDgate 76. The ANDgate 76 produces pulses which are applied to the ANDgate 77 similarly to the operation of the ANDgate 71.

Referring to FIG. 9, V-β is the lower limit reference voltage at the inverting input of thecomparator 70 and V+β is the higher limit reference voltage at the non-inverting input of thecomparator 75. Therefore, thecomparator 70 produces the high level output at the engine speed N-α corresponding to the voltage V-β and thecomparator 75 produces the high level output at the engine speed N+α corresponding to the voltage V+β. Accordingly, there is provided a non-operation engine speed zone α+α, one half on each side of a desired idling speed N.

The pulses from thereed switch 40 are applied to the one-shot multivibrator, comprising the EXCLUSIVE-OR gate 111 andinverters 112 and 114, etc., of the vehicle speedsignal generating circuit 56. The one-shot multivibrator generates two pulses per one input pulse, that is per one on-off operation of theswitch 40. The pulses are smoothed by theresistor 101 and thecapacitor 102, so that a voltage proportional to the vehicle speed is applied to the inverting input of thecomparator 103. If the NORgate 118 produces a high level output, theanalogue switch 116 is turned on. Thus, the non-inverting input of thecomparator 103 is applied with a low voltage. When the vehicle speed signal exceeds a predetermined input voltage at the inverting input of thecomparator 103, output of thecomparator 103 changes from a high level to a low level. The changing of the output is selected to occur at a low vehicle speed, for example at 8 Km/h.

When the actuatorcontact sensing switch 106 is off, that is thelever 21 is disengaged from therod 23, thetransistor 109 is turned on, so that the actuator contact signal on thelead 9 is at a low level. However, if the actuatorcontact sensing switch 106 is on, thetransistor 109 is turned off, so that the actuator contact signal goes to a high level.

It will be seen that ANDgates 72 and 77 produce a high level output when the input applied from the ANDgate 59 is at a high level and that the ANDgate 59 produces a high level output when all inputs applied from the vehicle speedsignal generating circuit 56 and the actuatorsignal generating circuit 57 are at high levels. The conditions are as follows:

(A) Vehicle speed is lower than a predetermined speed:

(B) Thelever 21 engages the end of therod 23.

Under these conditions, the ANDgate 59 produces a high level output to open the ANDgates 72 and 77 for controlling the idling speed.

When the engine idling speed is lower than the speed N-α (FIG. 9), thecomparator 70 produces a high level output which actuates the ANDgate 71 to produce pulses. The pulses are applied to the base of the transistor 98 of theactuator control circuit 55 through the ANDgate 72 to periodically turn on the transistor. As a result, thetransistors 88 and 91 are turned on, so that current passes through thetransistor 88,motor 38, andtransistor 91, which causes the rotation of themotor 38 in one direction. The rotation of the motor is transmitted to therod 23 through thegears 36, 35, 33 and 32, so that therod 23 projects to push thelever 21. Thus, thethrottle valve 18 is rotated to open the induction passage to increase the engine idling speed.

When the engine idling speed exceeds the speed N+α, thecomparator 75 produces a high level output, so that thetransistor 99 of theactuator control circuit 55 is turned on in a similar manner to thecircuit 51. Thus, thetransistors 89 and 90 are turned on and themotor 38 rotates in reverse, so that therod 23 is retracted. Thethrottle valve 18 is rotated by thespring 19 to close the passage to decrease the engine idling speed. Thus, the engine idling speed is automatically maintained at the desired idling speed N.

Conditions when the above-described speed control is not effected are as follows:

(C) Vehicle speed is higher than a predetermined value and the output of thecircuit 56 changes to a low level:

(D) Engine is accelerated and the actuatorcontact sensing switch 106 is turned off, so that the actuator signal of the actuatorsignal generating circuit 57 goes to a low level.

Under at least one of these conditions, the output of the ANDgate 59 goes to a low level thereby stopping the motor control operation.

Describing the operation for detecting the neutral state with reference to FIG. 8, when the vehicle is driven by theengine 1, the vehicle speed varies at a ratio dependent on the transmission ratio of the speed gear selected in the transmission provided for the engine. FIG. 10 shows the relationship between the engine speed and the vehicle speed. It will be seen from the graph that zone A and zone B are ranges in which the vehicle speed is not proportional to the engine speed. In other words, zones A and B are out of the transmission ratio range, that is, engine-speed to vehicle-speed ratio is excessively high in the zone A and is excessively low in the zone B. Therefore, when the point determined by the engine speed to vehicle speed ratio is in the zone A or zone B, the engine output to transmission system is cut off, that is the transmission is in the neutral state.

In the disclosed system, the neutral state of zone A is detected by counting ignition pulses from the engine speed sensor comprising theignition coil 5 in the period of each pulse from the vehicle speed sensor 13 (reed switch 40). When the number of the ignition pulses is greater than a predetermined number, the system judges that the point determined by the engine speed to vehicle speed ratio is in the zone A as will be described hereinafter.

Ignition pulses shaped by thewaveform shaping circuit 161 are counted by thecounter 120. When thecounter 120 counts fourteen pulses, Q₁, Q₂, Q₃ outputs go to high levels causing the ANDgate 122 to produce an output to set the flip-flop 123.

On the other hand, the vehicle speed signal pulse from thereed switch 40 operates the one-shot multivibrator 133 to produce an output pulse having a predetermined width. The output pulse of the one-shot multivibrator 133 is differentiated by thecapacitor 141 andresistor 143. The triangular pulse produced by differentiating the positive-going voltage of the output pulse is applied to the ANDgate 125 to reset the flip-flop 124. In addition, the negative-going voltage of the output pulse of the one-shot multivibrator 133 is also differentiated by the capacitor 140 andresistor 142 and resets thecounter 120 and the flip-flop 123. Thus, thecounter 120 and the flip-flop 123 are reset with a time lag corresponding to the width of the output pulse.

Describing the operation of the first neutral state judging circuit 121, under the power transmitting condition, the engine speed signal and the vehicle speed signal are in a proportional relation. Therefore, thecounter 120 is reset by the output signal of the one-shot multivibrator 133 before the counter counts fourteen ignition pulses from the engine speed sensor comprising theignition coil 5. When the transmission is in the neutral state, thecounter 120 counts fourteen pulses causing the ANDgate 122 to produce an output which sets the flip-flops 123 and 124. Thus, the flip-flop 124 produces an output a as a zone A signal. When the output pulse of the one-shot multivibrator 133 goes to a low level and the reset is produced at the capacitor 140, thecounter 120 and flip-flop 123 are reset. Although the Q output of the flip-flop 123 goes to a high level, the other input of the ANDgate 125 is at low level. Accordingly, the flip-flop 124 continues to produce the zone A output a. When the output of the one-shot multivibrator 133 goes to a high level and the reset pulse is applied to the ANDgate 125, the flip-flop 124 is reset and the zone A output signal a disappears. On the other hand, thecounter 120 starts to count after the reset pulse from the capacitor 140.

The operation of the zone B neutral state detecting circuit is similar to that of the zone A neutral state detecting circuit. In the system for the zone B, the zone B is detected by counting the vehicle speed signal pulses during an ignition pulse. When thecounter 126 counts three pulses, the Q₀ and Q₁ outputs go to high levels causing the ANDgate 128 to produce a high level output. On the other hand, when an ignition pulse is applied to the one-shot multivibrator 145 through the flip-flop 144, an output is produced for a predetermined period. The output is differentiated bycapacitors 152, 153 andresistors 154, 155, the outputs of which reset thecounter 126, and the flip-flops 129, 130. When the reset signals are generated after the three count of thecounter 126, a zone B neutral state signal b is produced from the Q output of the flip-flop 130. If the reset operations are performed before the three count, the zone B neutral state signal b is not generated. In other words, the engine speed is proportional to the vehicle speed.

Thus, in accordance with the present invention, the neutral state of the transmission can be reliably detected.

In the system of FIG. 7, the neutral signals are used for the automatic, engine speed control. The A or B zone neutral state signals a or b are applied to the NORgate 118, so that the NOR gate produces a low level output. Therefore, theanalogue switch 116 is off, so that the reference voltage at the non-inverting input of thecomparator 103 is raised to a high value. Thus, the automatic, engine speed is carried out in a higher vehicle speed range than the vehicle speed range at the idling state (below 8 km/h).

Claims (7)

What is claimed is:

1. A system for detecting a neutral state of a transmission for an engine mounted on a vehicle, the transmission having a plurality of transmission ratios dependent on change speed gears of said transmission, the system comprising

an engine speed detecting means for producing an engine speed output in proportion to the engine speed;

a vehicle speed detecting means for producing a vehicle speed output in proportion to the vehicle speed;

an operation circuit means for judging whether the ratio of said engine speed output to said vehicle speed output is in a range of the transmission ratios dependent on said transmission;

said operation circuit means producing at least one neutral state output when said ratio is outside of said range of said transmission ratios, thereby detecting the neutral state of the transmission,

said engine speed detecting means produces output pulses and said vehicle speed detecting means produces output pulses,

said operation circuit means comprises,

counter means for counting said output pulses from one of said detecting means and for producing a first signal when counting a predetermined number of the pulses before said counter means is reset,

first circuit means responsive to each of said output pulses from the other detecting means for producing a second signal for resetting said counter means thereby determining a respective period of operation of said counter means, and

second circuit means including gate means responsive to said first and second signals for producing said neutral state output in dependency on the result of said counting during said period.

2. The system for detecting a neutral state of a transmission for an engine in accordance with claim 1, wherein

said second circuit means including gate means comprises,

a first RS flip-flop and a second RS flip-flop connected in series from said counter means,

said first circuit means is connected to said counter means and said first RS flip-flop for simultaneous resetting,

said gate means is connected to the reset input of said second RS flip-flop and in cooperation with said first circuit means is for resetting said second RS flip-flop with delay after said counter means and said first flip-flop are reset.

3. The system for detecting a neutral state of a transmission for an engine in accordance with claim 2, wherein

said gate means has one input connected to the Q output of said first RS flip-flop and another input connected to said first circuit means.

4. The system for detecting a neutral state of a transmission for an engine in accordance with claim 2, wherein

said first circuit means is a one-shot multivibrator and includes an inverter at the output of said one-shot multivibrator,

said another input of said gate means is operatively connected to one side of said invertor and the other side of said invertor is operatively connected to said counter means and said first RS flip-flop for the resetting.

5. A system for detecting a neutral state of a transmission for an engine mounted on a vehicle, the transmission having a plurality of transmission ratios dependent on change speed gears of said transmission, the system comprising

an engine speed detecting means for producing an engine speed output in proportion to the engine speed;

a vehicle speed detecting means for producing a vehicle speed output in proportion to the vehicle speed;

an operation circuit means for judging whether the ratio of said engine speed output to said vehicle speed output is in a range of the transmission ratios dependent on said transmission;

said operation circuit means is for producing two neutral state outputs when said ratio is outside of said range of said transmission ratios, thereby detecting the neutral state of the transmission,

said engine speed detecting means produces output pulses and said vehicle speed detecting means produces output pulses,

said operation circuit means comprises,

a first counter means for counting said output pulses from said engine speed detecting means and for producing a first signal when counting a predetermined number of the pulses,

a first circuit means responsive to each of said output pulses respectively from the vehicle speed detecting means and for producing a second signal for determining a respective period of operation of said counter means,

a second counter means for counting said output pulses from said vehicle speed detecting means and for producing a third signal when counting a predetermined number of said output pulses from said vehicle speed detecting means,

a second circuit means responsive to each of the output pulses from said engine speed detecting means for producing a fourth signal for determining a respective period of operation of said second counter means, and

third and fourth circuit means including gate means responsive to said first to fourth signals for producing said neutral state outputs in dependency on the result of said counting during said periods, respectively.

6. A system for detecting a neutral state of a transmission for an engine mounted on a vehicle, the transmission having a plurality of transmission ratios dependent on change speed gears of said transmission, the system comprising

an engine speed detecting means for producing an engine speed output in proportion to the engine speed;

a vehicle speed detecting means for producing a vehicle speed output in proportion to the vehicle speed;

an operation circuit means for judging whether the ratio of said engine speed output to said vehicle speed output is in a range of the transmission ratios dependent on said transmission;

said operation circuit means for producing at least one neutral state output when said ratio is outside of said range of said transmission ratios, thereby detecting the neutral state of the transmission,

an idling speed control circuit for controlling the idling speed of said engine under conditions of vehicle speed,

said idling speed control circuit includes a comparator having one input operatively connected to said vehicle speed detecting means,

means connected to said operation circuit means for receiving said neutral state output and for increasing the voltage applied to another input of said comparator when said neutral state output occurs so as to increase the range of the idling speed control to higher vehicle speeds,

said comparator is an operational amplifier, said one input thereof is an inverting input and said another input is a non-inverting input, and

said means connected to said operation circuit means includes a switch means.

7. The system for detecting a neutral state of a transmission for an engine in accordance with claim 6, wherein

said means connected to said operation circuit means includes a logic circuit means connected to said switch means.

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